This article was originally featured on MIT Press. This article is adapted from Jeffrey McKinnon’s book “Our Ancient Lakes.“
Glance around any social event and it’s obvious that people, like all living things, vary in most any trait one can see or measure. And with our newfound ability to sequence entire genomes from thousands of species, we are learning that even more variation is hidden in our DNA. Working out how all this variation persists has been one of the great challenges of evolutionary biology.
So it’s refreshing and even surprising that in an age of automated DNA sequencing and artificial intelligence, important progress on this longstanding problem has been coming from careful field studies of a peculiar fish from a remote ancient lake.
Our story begins in 1954, on the shores of Lake Tanganyika in then Belgian Congo, with a little paper about fish diets titled “A Curious Ecological ‘Niche’ among the Fishes of Lake Tanganyika.”
The authors, biologists Georges Marlier and Narcisse Leleup, describe a little-studied species of cichlid fish. According to their findings, adults subsist mainly on the scales of other fish, which they tear off their living prey with fearsome teeth. Marlier and Leleup note that the individuals they held in an aquarium would not eat “earthworms, fish powder or insects” or anything else they presented other than the scales of live fish.
A few decades later, in one of the first studies to look carefully at evolution in this and several related species, the eminent Harvard fish biologist Karel Liem and his coauthor Donald Stewart investigated the mechanics of how these fish evolved to feed on scales, with a special focus on their jaws and teeth. They described a new species with notably extreme laterality (a particularly strong twisting of the head toward the left side or the right) and proposed that asymmetry in the shape of these fish’s skulls was associated with their…
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